Nanoparticles are of significant current interest because of the novel properties which arise from the small size and high ratio of surface atoms. Nanoparticle based technologies take advantage of the fact that materials built from particles less than a critical length (Poole, C. P. J.; Owens, F. J. Introduction to Nanotechnology; Wiley-Interscience: Hoboken, 2003) display unique chemical and physical properties. Many nanoscale materials manifest unusual mechanical, chemical, magnetic, and optical properties which can be utilized in applications demanding improved or specialized performance. J. H. Adair, et al., Mat'ls Sci. & Eng'g R- Reports, 23, 139 (Aug. 20, 1998).
Not only do these unusual properties arise on the nanoscale, but they also often depend heavily on the size, shape, and composition of the nanoparticles. The size-dependent properties of nanoparticles allows one to engineer them to have specific functions, such as in catalysts (Pal, B.; Torimoto, T.; Iwasaki, K.; Shibayama, T.; Takahashi, H.; Ohtani, B. J Phys. Chem. B 2004, 108, 18670; Maillard, F.; Schreier, S.; Hanzlik, M.; Savinova, E. R.; Weinkauf, S.; Stimming, U. Phys. Chem. Chem. Phys. 2005, 7, 385; Arenz, M.; Mayrhofer, K. J. J.; Stamenkovic, V.; Blizanac, B. B.; Tomoyuki, T.; Ross, P. N.; Markovic, N. M. J. Am. Chem. Soc'y 2005, 127, 6819), quantum dots for optical properties (Nakashima, P. N. H.; Tsuzuki, T.; Johnson, A. W. S. J. Appl. Phys. 1999, 85, 1556; Mahamuni, S. Solid State Physics, Proceedings of the DAE Solid State Physics Symposium, 41st, Kurukshetra, India, Dec. 27-31, 1998 1999, 33), and medical applications (Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013). Thus, controlling the size and size distribution can provide opportunities to tune the special characteristics of nanomaterials for chosen applications.
The preparation of monodisperse metal particles is necessary to study the effects of size on their novel applications including size-dependent conduction of electrons in Ag nanoparticles (Drachev, V. P.; Buin, A. K.; Nakotte, H.; Shalaev, V. M. Nano Lett. 2004, 4, 1535) and size dependent oxidation with Au catalysts (Valden, M.; Lai, X.; Goodman, D. W. Science (Washington, D. C.) 1998, 281, 1647; Haruta, M. Catalysis Today 1997, 36, 153). Moreover, monodisperse nanoparticles are also critical in the production of high quality ordered arrays and ordered thin films. Stowell, C.; Korgel, B. A. Nano Lett. 2001, 1, 595; Shah, P. S.; Sigman, M. B., Jr.; Stowell, C. A.; Lim, K. T.; Johnston, K. P.; Korgel, B. A. Advanced Materials (Weinheim, Germany) 2003, 15, 971; Shah, P. S.; Novick, B. J.; Hwang, H. S.; Lim, K. T.; Carbonell, R. G.; Johnston, K. P.; Korgel, B. A. Nano Lett. 2003, 3, 1671; Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379.
However, despite the widespread interest and investigation of ligand-stabilized nanoparticles over the last decade, relatively little improvement has been made to the time-consuming and solvent-intensive techniques employed for isolating monodisperse particle populations of ligand-stabilized particles. C. H. Fischer, H. Weller, L. Katsikas, A. Henglein, Langmuir 5, 429 (1989); T. Vossmeyer, et al., J. Phys. Chem. 98, 7665 (1994).
There are numerous methods to produce metal nanoparticles, including simple solution-based techniques such as reverse micelle synthesis (Cason, J. P.; Roberts, C. B. J. Phys. Chem. B 2000, 104, 1217; Kitchens, C. L.; McLeod, M. C.; Roberts, C. B. J. Phys. Chem. B 2003, 107, 11331) and two-phase arrested precipitation methods (Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc'y, Chem. Comm. 1994, 801). While these particular solution-based techniques are attractive due to their simplicity, they often result in the synthesis of particle sizes with a wide size range (e.g., 1 to 20 nm). Therefore, post-synthesis processing is required to further refine the size distribution to a desired narrow monodisperse range. A major current thrust of research is now being placed on post-synthesis particle manipulation for application to fields such as catalysis, optical systems, electronic devices, and sensors. Brust, M.; Keily, C. J., Colloid. Surface. A 2002, 202, 175-186; Collier, C.P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371-404.
A variety of post-synthesis techniques have been developed to narrow size distributions including the use of liquid antisolvents (Korgel, B. A.; Fullam, S.; Connolly, S.; Fitzmaurice, D. J. Phys. Chem. B 1998, 102, 8379; Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Amer. Chem. Soc'y 1993, 115, 8706; Sigman, M. B., Jr.; Saunders, A. E.; Korgel, B. A. Langmuir 2004, 20, 978) to selectively control precipitation, isoelectric focusing electrophoresis (IEF) (Arnaud, I.; Abid, J.-P.; Roussel, C.; Girault, H. H. Chem. Comm. (Cambridge, United Kingdom) 2005, 787), and chromatography techniques (Siebrands, T.; Giersig, M.; Mulvaney, P.; Fischer, C. H. Langmuir 1993, 9, 2297), to name a few. As an example, Sigman et al. (Sigman, M. B., Jr.; Saunders, A. E.; Korgel, B. A. Langmuir 2004, 20, 978) used ethanol as an antisolvent and centrifugation to size-selectively precipitate and separate a polydisperse dispersion of silver nanoparticles capped with dodecanethiol ligands into monodisperse particle fractions.
In these antisolvent nanoparticle precipitation techniques, ligand-capped particles are first dispersed in solution where the interaction between the solvent and the ligand tails provides enough repulsive force to overcome the inherent van der Waals attraction between the particles that would otherwise result in agglomeration and precipitation. Through the addition of an antisolvent, the resultant poorer solvent mixture interacts less with the ligand tails than the pure solvent did, thereby rendering a lesser ability of the solvent/antisolvent mixture to disperse the particles. Larger particles possess greater interparticle van der Waals attractions and, therefore, precipitate first upon worsening solvent conditions followed by subsequent precipitation of the smaller-sized particles with further addition of antisolvent. Applying centrifugation then provides external force to accelerate the precipitation process. Repetition of this antisolvent/centrifugation method on the separated particles can result in narrow particle size distributions, σ<5%; however, the whole process is both solvent and time intensive. It is also difficult to obtain an a priori desired particle size through this separation process in a repeatable manner simply by changing the composition of the liquid antisolvent/solvent pair.
Research in the area of compressed and supercritical fluid solvents has shown that the pressure- and temperature-tunable solvent properties in these systems provides a means to control the size of nanoparticles that can be synthesized and/or dispersed. Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 9574; Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2002, 106, 12178; Kitchens, C. L.; Roberts, C. B. Indus. & Eng'g Chem. Res. 2004, 43, 6070; Clarke, N. Z.; Waters, C.; Johnson, K. A.; Satherley, J.; Schiffrin, D. J. Langmuir 2001, 17, 6048. Shah et al. demonstrated the size-selective dispersion of dodecanethiol-coated nanoparticles in supercritical ethane by density tuning. Shah, P. S.; Holmes, J. D.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2002, 106, 2545. They illustrated that with the change in solvent density, the dispersable particle size could be adjusted where the largest particle sizes were dispersed at the highest pressure. However, ethane is a feeble solvent and very high pressures of around 414 bar were required to disperse particles of only 3.7 nm in size. Efficient solvent-based separation techniques for a wide range of nanoparticle sizes would require better solvent strength than these supercritical solvents are able to provide at significantly lower pressures.
To overcome these limitations and to provide improved control over size selective precipitation, we developed a method of the current invention which utilizes gas expanded liquids (GELs). Among their many applications, GELs have been used as tunable reaction media (Wei, M.; Musie, G. T.; Busch, D. H.; Subramaniam, B. J. Amer. Chem. Soc'y 2002, 124, 2513; Thomas, C. A.; Bonilla, R. J.; Huang, Y.; Jessop, P. G. Can. J. Chem. 2001, 79, 719; Xie, X.; Liotta, C. L.; Eckert, C. A. Ind'l & Eng'g Chem. Res. 2004, 43, 7907), as adjustable solvents for separations (Gallagher, P. M.; Coffey, M. P.; Krukonis, V. J.; Klasutis, N. ACS Symp. Series 1989, 406, 334; Eckert, C. A.; Bush, D.; Brown, J. S.; Liotta, C. L. Ind'l & Eng'g Chem. Research 2000, 39, 4615; Chen, J.; Zhang, J.; Liu, D.; Liu, Z.; Han, B.; Yang, G. Coll. and Surfaces, B: Biointerfaces 2004, 33, 33; Xie, X.; Brown, J. S.; Joseph, P. J.; Liotta, C. L.; Eckert, C. A. Chem. Comm. (Cambridge, United Kingdom) 2002, 1156), in the switching of fluorous compound solubilities (Jessop, P. G.; Olmstead, M. M.; Ablan, C. D.; Grabenauer, M.; Sheppard, D.; Eckert, C. A.; Liotta, C. L. Inorg. Chem.y 2002, 41, 3463), and in gas antisolvent (GAS) precipitation techniques for organic and polymer microparticle formation (Randolph, T. W.; Randolph, A. D.; Mebes, M.; Yeung, S. Biotech. Prog. 1993, 9, 429).
Gas expanded liquid systems provide a wide range of solvent properties (from liquid-like to gas-like) that are widely tunable with simple adjustments in gas pressure thereby providing further opportunity for nanoparticle precipitation and separation. McLeod, M. C.; Anand, M.; Kitchens, C. L.; Roberts, C. B. Nano Lett. 2005, 5, 461. For example, Han and coworkers precipitated nanoparticles from Aerosal OT (sodium bis(2-ethylhexyl)sulfosuccinate) (AOT) reverse micelles in liquid isooctane using pressurized CO2 as an antisolvent. Liu, D.; Zhang, J.; Han, B.; Chen, J.; Li, Z.; Shen, D.; Yang, G. Coll. and Surf., A: Physicochem. Eng'g Aspects 2003, 227, 45; Zhang, J.; Han, B.; Liu, J.; Zhang, X.; He, J.; Liu, Z.; Jiang, T.; Yang, G. Chemistry—A European Journal 2002, 8, 3879; Zhang, J.; Han, B.; Liu, J.; Zhang, X.; Yang, G.; Zhao, H. J. Supercrit. Fluids 2004, 30, 89.
Full utilization of nanoparticles for various applications requires the ability to effectively process and accurately maneuver particles onto surfaces or support structures. This encompasses areas ranging from particle size selection to accurate manipulation of particles onto surfaces such as assembled nanoparticle thin films, arrays, and superstructures. This is often performed by evaporating a liquid solution containing dispersed solvated/stabilized nanoparticles to leave ordered lattice structures. Collier, et al. 1998; Sigman, M.B., Jr.; Saunders, A. E.; Korgel, B. A. Langmuir 2004, 20, 978-983. However, wetting effects and surface tensions inherent with liquid/vapor interfaces can lead to random film defects such as interconnected lattices and areas of inconsistent particle deposition and assembly.
Preparation of particles for surface deposition is commonly performed using wet chemical methods such as arrested precipitation techniques (Sigman, et al., 2004; Brust, M., Walker, M., Bethell, D., Schiffrin, D. J., Whyman, R. J. Chem. Soc. Chem. Comm. 1994, 801-802) or reverse micelle methods (Petit, C., Lixon, P., Pileni, M. P. J. Phys. Chem. 1993, 97, 12974-12983). After particle synthesis, the particles are generally separated and cleaned from the synthesis solution using a liquid antisolvent to precipitate the particles. Nanoparticle dispersal in a solvent arises from the solvation of ligand tails attached to the nanoparticles. When these tails are well solvated, the particles can be effectively dispersed in the solvent. Similarly, addition of an antisolvent with regard to the ligand tails functions to lower the dispersibility of the ligand-coated nanoparticles.
Han and co-workers (Liu, D., Zhang, J., Han, B., Chen, J., Li, Z., Shen, D., Yang, G. Colloid. Surface. A 2003, 227, 45-48; Zhang, J., Han, B., Liu, J., Zhang, X., Liu, Z., He, J. Chem. Commun. 2001, 2724-2725; Zhang, J., Han, B., Liu, J., Zhang, X., He, J., Liu, Z., Jiang, T., Yang, G. Chem.-Eur. J. 2002, 8, 3879-3883; Zhang, J., Xiao, M., Liu, Z., Han, B., Jiang, T., He, J., Yang, G. J. Colloid Interf. Sci. 2004, 273, 160-164) have shown the ability to use CO2 as a gas antisolvent to recover nanoparticles of zinc sulfide, silver, and titanium dioxide from reverse micelles in isooctane. Such a technique is feasible because compressed gases such as CO2 can dissolve into organic liquids and expand the liquid volume significantly while also altering the liquids' solvation characteristics. Kordikowski, A., Schenk, A. P., Van Nielen, R. M., Peters, C. J. J. Supercrit. Fluids 1995, 8, 205-216. These liquid solutions are referred to as gas-expanded liquids and have been used in gas antisolvent (GAS) techniques for the precipitation of organic compounds from organic solutions (Gallagher, P. S., Coffey, M. P., Krukonis, V. J., Klasutis, N., In Supercritical Fluid Science and Technology, Johnston, K. P., Penninger, M. L., Eds., 1989, Vol. 406, pp. 334-354), for reaction tuning (Thomas, C. A., Bonilla, R. J., Huang, Y., Jessop, P. G. Canadian J. of Chem. 2001, 79, 719-724; Wei, M., Musie Ghezai, T., Busch Daryle, H., Subramaniam, B. J. J. Am. Chem. Soc. 2002, 124, 2513-2517), and for the switching of fluorous compound solubility in organic solvents (Jessop, P. G., Olmstead, M. M., Ablan, C. D., Grabenauer, M., Sheppard, D., Eckert, C., A. Liotta, C. L. Inorg. Chem. 2002, 41, 3463-3468), among others. As the CO2 is increasingly added to the organic solution, any solutes present which are not solvated by the gas-expanded liquid are forced to precipitate. Xie, X., Brown, J. S., Joseph, P. J., Liotta, C. L., Eckert, C. A. Chem. Commun. 2002, 1156-1157; Reverchon, E. J. Supercrit. Fluids 1999, 15, 1-21.
In some cases the capillary forces present at the liquid/vapor interface of a drying liquid can function advantageously to organize particles into ordered nanocrystal superlattices or to position nanosized structures on a substrate. Lin, X. M.; Jaeger, H. M.; Sorensen, C. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3353-3357; Korgel, B. A., Fitzmaurice, D. Phys. Rev. Lett. 1998, 80, 3531-3534; Cui, Y., Bjoerk, M. T., Liddle, J. A., Soennichesen, C., Boussert, B., Alivisatos, A. P. Nano Lett. 2004, 4, 1093-1098. For example, Cui et al., 2004, recently utilized the capillary force at the solution interface as a means of positioning sub-10-nm-size particles and tetrapod nanostructures upon a lithographically patterned surface.
However, the dewetting process can give rise to undesirable features in the deposition of nanoparticle films over large areas. Capillary forces and high surface tensions at the liquid/vapor interface commonly lead to nanoparticle deposition features such as isolated islands, percolating domains, locally high particle concentrations, ring-like structures, and uneven surface coverage. Lin, et al. 2001; Korgel, et al. 1998; Ohara, P. C., Gelbert, W. M. Langmuir 1998, 14, 3418-3424; Ohara, P. C., Leff, D. V., Heath, J. R., Gelbert, W. M. Phys. Rev. Lett. 1995, 75, 3466-3469; Motte, L., Billoudet, F., Lacaze, E., Douin, J., Pileni, M. P. J. Phys. Chem. B 1997, 101, 138-144; Giersig, M., Mulvaney, P. Langmuir 1993, 9, 3408-3413. This phenomenon is effectively demonstrated by work done by Ohara et al., 1998, which showed that as holes open in the surface of a thin solvent film during evaporation, the dewetting liquid interface pulls the particles along the rim of the opening hole. The particles eventually become pinned to the surface and the liquid interface moves over the particles, leaving behind ring-like nanoparticle arrays.
In order to overcome surface wetting instabilities, Shah et al. (Shah, P. S., Novick, B. J., Hwang, H. S., Lim, K. T., Carbonell, R. G., Johnston, K. P., Korgel, B. A. Nano Lett. 2003, 3, 1671-1675) recently used evaporating liquid carbon dioxide, rather than organic liquids, as the medium for particle deposition. Since CO2 does not exhibit dewetting instabilities, it was used to study the kinetics of nanocrystal monolayer formation when fluorooctyl methacrylate thiol-coated particles were deposited at different CO2 evaporation rates. The fluorooctyl methacrylate thiols were required to induce dispersal of the gold nanoparticles in CO2. Unfortunately, it was reported that, even with the CO2-philic fluorinated ligands, the concentration of particles in liquid CO2 was limited because of their low dispersibility in liquid CO2.
Despite the inability of CO2 to stabilize and easily disperse nanoparticles, it is still an excellent process solvent for nanomaterial features due to its low viscosity and low surface tension. Johnston, K. P., Shah, P. S. Science 2004, 303, 482-483; Shah, P. S., Hanrath, T., Johnston, K. P., Korgel, B. A. Phys. Chem. B 2004, 108, 9571-9587; Holmes, J. D., Lyons, D. M. Ziegler, K. J. Chem.-Eur. J. 2003, 9, 2144-2150; Blackburn, J. M., Long, D. P., Cabanas, A., Watkins, J. J. Science 2001, 294, 141-145. The work of Shah et al., 2003, advantageously used the properties of liquid CO2 to form gold nanocrystal monolayers.
Further improvement of the properties of CO2 as a process solvent can be achieved by operation under supercritical fluid conditions, where it possesses low interfacial tensions, high diffusivities, and superior surface wetting properties. Supercritical CO2 is an ideal solvent for processing these nanoparticle films due to its low interfacial tensions, excellent wetting of surfaces, and the avoidance of phase transitions when transitioning from liquid-like densities to vapor-like densities. In addition, CO2 is non-toxic, non-flammable, and inexpensive.